If I were you I'd write up an email describing what has happened and email it to all of the chapter presidents, advisors, cpos, presidents of alumnae groups across the country. And if you can find email addys for influential/famous sigma kappas i'd send it to them too (it always helps to have friends in high places). In that email be sure to describe greek chat (all about it, not just that this is somewhere where we can disuss what has happend openly) and ask them all to come and be registered members. Tell the person who recieves the letter to forward/pass the news along to her chapter. This will be a lot of work, but it could pay off for you girls in the end. Not everyone will be receptive to your opinions, but I'm sure there are plenty of chapters out there that are willing to stand up behind you. You may not get the best response at first because it is summer, but sooner or later word will spread.
And oh yeah, have every member of your chapter attend convetion if they can. Make sure the ones that are there are rather chatty.

But don't go overboard....you don't want to make it sound like you are whining. Show everyone at convention how mature you are about the situation. Once you talk to them they will probably go tell someone else, and by the end of convention everyone will know (If they didn't already by wondering why the host chapter wasn't active anymore).

Knowledge is key, and hopefully with everyone knowing nationals will cave under the pressure, make an apology, and you will get to have you chapter come back actively! Best of luck!
SLAM,
Jessica
PS--Is your local alumnae chapter/chapter alumnae helping you with all of this? If not, ask them to! And if emailing people doesn't work (someone may think its junk mail/not believing its true), send letters if you can afford it! That will get their attention!